Neuromuscular electrical stimulation (“NMES”), which is also referred to as powered muscle stimulation, functional muscle stimulation, electrical muscle stimulation, is a known technology with many therapeutic uses, including pain relief, prevention or retardation of disuse atrophy, and improvement of local blood circulation. NMES is typically delivered as an intermittent and repeating series of short electrical pulses delivered transcutaneously by surface electrodes that are attached to a person's skin. The electrical pulses are delivered to muscle tissue and/or a muscle nerve to induce muscle contraction. The electrodes may be secured to the skin using straps, adhesives, or other mechanisms, and often contain a coupling layer composed of hydrogel that is capable of enhancing the efficiency of energy transfer from the electrode to the skin and underlying tissues.
A group of persons who could potentially show large benefit from NMES therapy are those who are immobilized or confined to bed rest. Periods of immobilization lead to muscle atrophy and weakness, and have severe effects on a person's physical capacity. Following immobilization, a previously active and functional person will typically require extensive physical therapy to reclaim their prior level of functionality. NMES may help these persons by preventing or retarding muscle atrophy during immobilization.
Critically ill patients comprise a subgroup of immobilized individuals. While virtually all of these patients are confined to bed rest, many are also suffering from conditions such as coma or are receiving interventions (such as mechanical ventilation) that generally require sedation and/or analgesia. Sedated or comatose patients are at a great risk for muscle atrophy because even simple voluntary movements (such as shifting arms/legs in bed or moving one's feet) are often not performed. Consequently, critically ill patients face long paths to recovery that are generally measured in months as opposed to days or weeks.
As part of the care for their acute illness, many critically ill patients receive I/V fluids, antibiotics, and other interventions. One common side effect of these medical treatments in immobilized patients is the development of tissue edema. Generally speaking, tissue edema occurs as bodily fluids accumulate in ‘the third space’, or the region outside of both cells and vessels. Edema is often caused by microvasculature leakage, and typically results in tissue swelling. The presence of edema will generally negatively affect the performance of NMES, in many cases limiting the ability of the technology to adequately induce muscle contraction. This is particularly true when attempting to stimulate deep-lying muscles, such as the quadriceps, hamstrings, gluteals, rectus abdominus, transversus abdominus, internal and external obliques, pelvic floor, multifidus, erector spinae, longissimus thoracis, diaphragm, using non-invasive electrodes placed upon the surface of the skin.
There are several mechanisms of action by which tissue edema may affect NMES therapy. Tissue swelling may increase the distance between the surface of the skin and underlying muscle, resulting in a lower current density that reaches deep target muscles. Additionally, excessive ionic fluid in tissues may decrease the electrical impedance of tissue, particularly in superficial regions. The decrease in impedance in superficial regions can act to ‘short-circuit’ skin electrodes. The lower impedance path in superficial tissue regions can also act as a mechanism to reduce the current density in deeper muscle tissues. The latter of these mechanisms may be the dominant factor associated with decreased NMES performance in edematous patients. Although previous work in the medical literature has noted that certain types of electrical stimulation may prevent the onset of local edema after traumatic injury, these therapies have not been shown to prevent or reduce widespread edema in cases involving non-traumatic or multi-factorial medical conditions.
Existing NMES devices described in the prior art do not have features or compensation mechanisms to address tissue edema. Because of this, these devices provide highly variable performance in and are of limited utility amongst patients suffering from this condition. In many edematous patients, it is not possible to reliably stimulate the contraction of deep muscles using surface electrodes and energy levels that fall within regulatory and governing body (ex. the US FDA, ANSI, and IEC) standards. Although the use of higher energy levels may increase NMES efficacy, increasing the amplitude of delivered energy (and thus the current density in tissue), increases the risk of burns, nerve and/or muscle damage, and other potential complications (as detailed by Prausnitz Advanced Drug Delivery Reviews 18:395-425, 2006 and Stecker et al Am J END Tech., 43:315-342, 2006, both of which are incorporated herein by reference). This is particularly true for the ‘short circuit’ condition, as large current densities will be present in superficial tissues. These factors and others limit the application of NMES therapy to edematous patients and to immobilized critically ill patients as a whole, a group that has been hypothesized to potentially benefit significantly from the therapy (Morris et al., Critical Care Clinics, 23:1-20, 2007—incorporated herein by reference). Delivery of safe and effective NMES therapy to immobilized critically ill patients would be facilitated by devices, systems, and methods designed to improve the performance of NMES, both in the presence of edema and in non-edematous persons and animals. Such devices, systems, and methods would allow for a larger patient cohort to receive the beneficial effects of a well-established medical therapy.
Short-duration superficial cooling may improve stimulation efficacy in patients both with and without peripheral tissue edema. For temperatures below 40° C., tissue impedance increases by about 2%/° C. (see Miklavcic et al., Electrical Properties of Tissues, Wiley Encyclopedia of Biomedical Engineering, 2006, incorporated herein by reference). Thus, when used with muscle stimulators operating in typical temperature ranges of use, surface cooling may increase the impedance of superficial tissues. Without wishing to be bound by any theory, it is believed that this may cause a greater percentage of delivered electrical energy to interact with deeper muscle tissues, facilitating more robust muscle contraction.
One application of reverse thermal gradients that has been described involves the combination of surface cooling with the targeted transcutaneous delivery of high energy radiofrequency (RF), optical, photo-acoustic, acoustic, infrared, electromagnetic, or other types of stimuli to tissues below the skin surface. Generally, these applications seek to significantly raise the temperature of tissues below the skin surface for the purposes of ablation, tissue (e.g., collagen) remodeling, or other dermatologic or therapeutic reasons. These applications seek to apply energy to target tissues non-invasively without raising temperatures in the skin and other superficial tissues to avoid damaging tissue not intended for treatment. The reverse thermal gradient assists this procedure by cooling superficial tissue without significantly cooling the deeper tissue that is intended to be treated by an increase in temperature. Accordingly, temperatures in superficial regions are kept below levels that would cause damage, even though a portion of the energy stimulus is absorbed in these regions.
A subset of thermal gradient applications described above use high amplitude RF or other forms of electromagnetic/electric energy to significantly raise temperatures in target tissue regions (e.g., hair follicles, collagen, etc.). To be effective, these treatments require temperatures in target regions of tissue to exceed about 43° C., with most applications requiring elevating tissue temperatures to about 60° C. or higher. Near these temperatures, moisture in cells and extracellular fluid is evaporated, resulting in increased tissue impedance with increased temperature. Reverse thermal gradients and surface cooling of tissues can assist energy delivery by forcing superficial tissue temperatures to remain only minimally elevated over normal body temperature, thus lowering the superficial tissue impedance (relative to the overheated tissues below), allowing for more energy to be delivered through the superficial tissue to the deeper target regions below.
For ablative, cosmetic, and other therapeutic procedures, muscle contraction is generally not induced by energy that is delivered to tissue. In virtually all cases, this is preferable, as muscle contraction in the region of desired treatment would complicate the intervention. For example, RF energy utilized by many devices is intentionally delivered in a frequency range, for example, about 100 to about 500 kHz, which is too high to elicit muscle contraction.
Additionally, in cosmetic, ablative, and therapeutic applications that use surface cooling to prevent skin burns, the reverse thermal gradient is applied at the anatomical location where energy transmits across the skin, or in larger regions that include the location at which energy is transmitted across the skin. These systems and methods utilizing the reverse thermal gradient are optimized for the energy amplitudes, frequency ranges, and temperature ranges that are common in these ablative, cosmetic, and therapeutic procedures. For muscle stimulators operating at relatively lower energy frequencies and amplitudes, with peak tissue temperatures near normal body temperature, there are drawbacks to lowering skin temperatures in the region where energy transmits across the skin. Doing so will significantly lower the efficiency of energy transfer into the body, markedly decrease the life span of surface stimulation electrodes, and decrease the overall effectiveness of the therapy.
Most muscle stimulators used in modern clinical settings are constant current (or voltage) stimulators, meaning that when tissue impedance increases, the stimulator device will increase the voltage (or current) amplitude of delivered energy (up to a predetermined limit) in an attempt to keep the electrical current (or voltage) delivered to a person constant. Without wishing to be bound by any theory, it is believed that this increase in voltage (or current) will increase energy loss and heat generation in skin electrodes. Although the risk of skin burns (generally a serious concern) may be partially reduced if the skin surface is pre-cooled, increased temperature of skin electrodes will degrade the performance of the electrodes. The most common modern-day skin electrodes used with NMES include a hydrogel coupling layer that serves as both an adhesive and a conductive (coupling) medium. These hydrogels may be composed of more than 50% water, and elevated temperatures will cause electrodes to dry prematurely, dramatically reducing reusability. This factor is particularly important in the ICU setting, where it is desirable to leave one set of electrodes in place for extended periods of time, as repeated placement and removal may cause skin trauma. Additionally, drying of hydrogel layers is a positive feedback phenomenon: as the conductive layer dries, skin/electrode impedance will increase further, causing even more heat generation at the skin, and potentially leading to the dangerous scenario of poor electrode contact due to reduced adhesive properties. This latter scenario is of serious concern, as electrodes with poor contact can cause skin burns very quickly, even when NMES is used in conjunction with surface cooling. Thus, devices employing surface cooling and temperature gradients used in the location of skin electrodes are accompanied by serious limitations if used in conjunction with NMES, since this technique raises tissue impedance in the skin electrode location. Specifically, surface cooling and temperature gradients in the location of the skin electrode(s) will typically not improve energy transfer efficiency to muscles, and may thus increase tissue impedance and decrease electrode performance in a manner that has little or no benefit for NMES.
Transcutaneous electrical nerve stimulators (“TENS”) is another type of therapy that has used skin surface cooling combined with transcutaneous energy delivery. Specifically, this therapy has sought to harness the pain relief effects of hot and cold temperatures applied to the skin, and combine them with pain relief effects of nerve stimulation. Although TENS units are typically not operated at sufficient amplitude to cause muscle contraction, muscle stimulation with TENS units is theoretically possible. TENS therapy also applies temperature gradients in the anatomical locations where energy is transmitted through the skin, or over large spans of anatomical areas that include the locations where energy is transmitted through the skin. As described herein, doing so with electrical muscle stimulation therapies significantly lowers the efficiency of energy transfer into the body, markedly decreases the life span of surface stimulation electrodes, and decreases the overall effectiveness of the therapy.
Existing NMES devices and technologies that are disclosed in the prior art are not suitable for use in patients with peripheral tissue edema and/or with other challenging patients. This may be for performance reasons or for practicality reasons (i.e., technology may be viable but configured in an embodiment that is prohibitive to use in challenging environments, for example the hospital environment). Disclosed within are devices, systems, and methods for improving muscle stimulation efficacy that meet both performance and usability criteria to make them acceptable for broad application to subjects in a wide spectrum of environments.